chore(receiver): packaging, k8s manifests, CI/release, docs

Dockerfile.receiver (CGO-free, /data volume), receiver image in CI and
tag releases, receiver binary in release archives, make build-receiver,
example k8s manifests (deployment/service/ingress/secret/pvc) under
deploy/k8s/receiver/, and README coverage including sender/receiver
token pairing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

deploy/k8s/receiver/pvc.yaml

@@ -0,0 +1,16 @@
+# TEMPLATE — backing storage for the SQLite database (RECEIVER_SQLITE_PATH
+# defaults to /data/exploredns-receiver.db in the container image). Not needed
+# when RECEIVER_MYSQL_DSN is set, but harmless to keep. Set storageClassName
+# if your cluster has no default class.
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: exploredns-receiver-data
+  namespace: exploredns-receiver
+spec:
+  accessModes:
+    - ReadWriteOnce
+  resources:
+    requests:
+      storage: 1Gi
+  # storageClassName: standard